Electric power steering systems (EPS) which derive a steering assist torque from an electric motor are being preferred over more conventional hydraulic power steering systems in recent times. As an electric power steering system relies on a car battery for powering the electric motor, the power output of the engine is not directly diverted from the engine for the power steering system, and the electric motor is powered only when a steering assist is required. Therefore, the fuel economy may be improved, and control by an ECU can be easily implemented.
In a conventional EPS, because the assist torque was entirely dependent on the steering torque of the vehicle operator, the inertia of the electric motor and frictions in various parts of the systems were not taken into account, and this caused some impairment in the steering feel. Based on such considerations, it was proposed to obtain a target steering torque (target centering torque) from a map based on the steering angle of the steering wheel, and to feedback control the assist torque according to the difference (steering torque difference) between the input steering torque (actual steering torque) obtained from a steering torque sensor and the target steering torque (see Patent Document 1).
Meanwhile, the steering geometry may consist of either Ackemann geometry where all of the wheels are caused to be steered around a common point and parallel geometry where all of the steerable wheels (typically front wheels) are steered by a same angle. By using Ackermann geometry, the side slip of the steerable wheels is controlled during low speed turn, and the wear of the tires and the rolling resistance to the tires can be reduced. However, in high speed turn, the direction of the cornering force of the tires may not coincide with that of the centrifugal force acting thereon. Therefore, in practice, the steering geometry of a typical four-wheel vehicle is set to be intermediate between Ackermann geometry and parallel geometry (or Ackermann ratio being anywhere between 30 and 70%), instead of pure Ackemann geometry (or Ackermann ratio being 100%) (see Patent Document 2).